In recent years, along with an increase in the scale of an optical transmission system, a size reduction in an optical module constituting the system has been demanded. While the optical module includes an optical modulator chip and electrical interfaces, the size of the electrical interface becomes a factor for determining the size of the optical module along with a size reduction of the optical modulator chip. The electrical interfaces are classified into an RF (Radio Frequency) pin to be connected to an RF electrode for a high-speed signal and a DC (Direct Current) pin to be connected to a DC electrode for controlling the optical modulator chip. More specifically, an RF signal is inputted from the RF pin provided on a side surface or bottom surface of a package and inputted into the RF electrode of the optical modulator chip via a relay board. Similarly, a DC signal is inputted from the DC pin provided on the side surface or bottom surface of the package and inputted into the DC electrode of the optical modulator chip via the relay board.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-048121 and Patent Document 2: Japanese Laid-open Patent Publication No. 2001-083908 are introduced as the Related Art Document.
Since four signals at 32 Gbps are inputted into the RF pin of the electrical interfaces, four RF pins are provided in the package. If the interval between these four RF pins is reduced in order to reduce the size of the optical module, however, crosstalk between the RF pins (channels) is increased. Therefore, in order to achieve a size reduction of the electrical interfaces, how much the interval between the DC pins can be reduced while keeping the interval between the RF pins at a level satisfying the characteristic impedance becomes important.
For example, twelve DC pins are arranged horizontally in a row on an FPC (Flexible Printed Circuit) provided in the package. This becomes a factor for complicating a size reduction in the optical module. In view of this, reducing the horizontal width of the FPC by arranging the DC pins in two rows is effective in reducing the size of the optical module. However, in a configuration where wirings extending from the DC pins in the outer row go through between the DC pins in the center-side (DC electrode side) row, the interval between the DC pins needs to be set large. This imposes a limitation on the size reduction of the optical module.
In view of this, if the wirings extending from the DC pins in the outer row are set so as to circumvent an outer side of the DC pins at the both ends in the center-side (DC electrode side) row, the optical module can connect the DC pins to the corresponding DC electrodes without increasing the interval between the DC pins. According to this configuration, however, the arrangement of the DC electrodes is different from the arrangement of the DC pins. Therefore, compatibility with the related DC electrode arrangement is not ensured. Moreover, since the FPC with the DC pins disposed thereon is extended toward the RF pin due to the above-described circumvention of the wirings, crosstalk between a DC signal and an RF signal is generated.